I "shoot" mostly birds and currently have a 6d with a Canon 500 f4 (version I) lens, and I always keep the Canon 1.4x extender on.

The above set up gets me excellent results. I enjoy the "hunting" required to get close enough to get good photos. However, on sunny days with water fowl, I can only get so close to the ducks and whatever.

Here is my question: Should I get a Canon 2x converter for longer range on sunny days or should I get a crop sensor camera for the extra reach?

Anybody have any experience with this? So basically what I would like to know is will the 6d with the 500mm lens and Canon 2x converter provide better overall IQ compared to, for example, a 70d with the 500 and 1.4x converter? And I am talking about sunny low ISO days here shooting birds.